The mortar began as a short, high-angle-fire weapon used in siege warfare. During World War I, it was well adapted to static trench warfare, and all participants had some version of the weapon. Mortars were the ideal infantry weapon, and their plunging fire rendered them most effective in trench warfare. Proper trench mortars, for which the two chief criteria were ease of use and portability, appeared in 1915. World War I mortars ranged in size from light 7.62 cm trench mortars firing a 9 lb projectile to heavy and difficult to move 340 mm mortars firing a projectile weighing 430 lbs. As infantry artillery, mortars could provide dedicated barrages for companies and battalions without recourse to true artillery units. Most mortar crews were infantrymen who were trained to use the weapon and were part of their unit. This meant that each unit within its parent could call for fire support when it was needed.

The mortar developed from the heavy trench mortar of World War I into the Brandt- and Stokes-type mortars of the 1930s. The latter were battlefield weapons that could be carried by a man and could deliver an effective shell without the time delay of conventional artillery. Set up and firing was a quick process, and in view of the relatively short range of mortars, fire control and correction were easy, even for frontline infantry.

The mortar, in comparison with artillery, has the benefit of simplicity, and it is cheaper to manufacture than a gun or howitzer. As with the howitzer, it has a high angle of fire. Its shell has excellent antipersonnel characteristics. As it descends from the near vertical, the shell, when detonated, produces a 360-degree blast and shrapnel effect, unlike the less effective low-angle gun shell. The mortar also can be used to overcome obstacles to direct fire, such as buildings, trenches, and hills. Lighter mortars are mobile, especially in terrain into which guns have difficulty penetrating, such as mountainous and wooded ground.

Technically, unlike the gun, the mortar has a low chamber pressure and a smooth barrel, factors that reduce wear on the barrel and thus prolong the effective accuracy and life of the weapon. As a tactical weapon, it is easy to conceal, and it can be fired from a trench, making it possible, although never really desirable, to have mortars in the front line.

In World War II, participants used light and medium mortars at the beginning, with much heavier mortars appearing later, particularly on the Eastern Front in the Soviet Union. Light mortars, of approximately 5 cm or 2-inch caliber, were used by, among others, the Germans, British, Japanese, and Americans. They were issued at the platoon level, the idea being to provide every platoon with a short-range weapon for immediate support. The mortar proved of some value for laying a small smoke screen, but it was soon realized that the high-explosive content of the shell was too small to be effective. The illumination shell, however, proved to be of great value at night, enabling a platoon to light up its front in an instant.

The Germans developed their World War I mortars along the lines of both the Brandt and the Stokes patterns to produce the 8 cm Granatwerfer 34, which was issued to all German army battalions in the support or heavy weapon companies that also controlled the heavy machine guns. The German employment of mortars in World War II was closely linked to their use of machine guns in both attack and defense, and their medium mortar teams gained a solid reputation for their work in supporting their infantry.

The Germans, however, regarded the Soviets as the masters of the mortar. The Soviets issued a series of 5 cm mortars, but these were overshadowed by their larger-caliber weapons. The 82 mm mortar Type M36 (and later, its successors) was the main infantry mortar, and although it fired only a 6.7 lb bomb, the number of mortars available in the Soviet battalion was so high that the effect on the Germans was often catastrophic.

The Soviets had a great liking for the mortar, and they began to bring in heavy mortars, increasing in caliber to 160 mm. These large-caliber weapons did not so much outrange their smaller brethren as outweigh them in the amount of explosive they could drop onto a target. Whereas the 82 mm shell weighed less than 6.71 lbs, the 160 mm shell weighed 89.7 lbs. Explosive fillers were 4.5 and 62 lbs, respectively, which meant that the explosive content of the latter was 14 times that of the former.

The Western Allies generally stuck to the 3-inch or 80 mm mortar during World War II because Allied artillery and air support meant that they did not have to develop new, light-weight methods of delivering shell on the battlefield. For the Germans, however, rocket-propelled shell, such as the 28/320 mm Schweres Wurfgerät 40 and 41 and the 180 mm Raketengranate 4331, meant easier, quicker, and cheaper shell manufacture and the means to deliver high weights of shell at less cost in money and in time on the battlefield.

Although strictly speaking not mortars, the Soviet Katyusha, or "Stalin Organ," rocket weapons caused the Germans to look carefully at small rocket-propelled shells. They already had the multibarreled Minenwerfer weapons, and soon, they dispensed even with barrels for firing these projectiles. The 28 cm and 32 cm Nebelwerfer 41 shells and many similar projectiles were fired from their packing cases. The shells' weight was such that they did not need sophisticated sighting systems, just an area to be hit. These latter-day mortars proved effective in battle, and they were the forerunners of today's Multiple Launch Rocket Systems (MLRS).

Development of the mortar in World War II followed two diverse paths. The first was the Allied weapons, where range, bomb, and accuracy improvements were seen as most important. These developments led to rifled-barrel mortars, proximity-fused and rod-detonated shells, and sighting systems nearly as complex as artillery gunsights. The conflict on the Eastern Front, however, led to greatly increased calibers and multibarreled mortars, essential against large masses of attacking infantry.